The types of furnaces used for heat treatment depend on the type of heat treatment, the range of composition of steel and its size etc. Furnace may be heated by solid fuel (pulverised fuel), oil, dirty gas (producer gas), clean gas, or by electricity; the choice being wide and governed by economics.
Furnace design also changes to meet the requirements such as atmosphere control, continuous operation instead of batch operation etc. The design and construction of furnaces of maximum efficiency require a thorough knowledge of the principles of heat transmission, characteristics of the fuels and the materials and refractories used in furnace construction.
A factor of importance in the design of furnaces has been the introduction and growing use of thin walls and light weight insulating bricks which have resulted in an increase in the speed of both heating and cooling because for furnaces of less sensible heat stored in the refractories.
However, if the cooling of the charge has to be very slow, better control is possible with well insulated furnaces. The need for a proper maintenance of pyrometric equipment to ensure accuracy of the heat treatment operation can’t be over-emphasised.
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For heat treatment purposes, generally either the gas- fired furnaces or electrical furnaces are used, because of their cleanliness of operation. In gas-fired furnaces, it is very easy to control the furnace atmosphere.
To obtain high temperatures in the furnace it is possible to preheat both gas and air. The parts to be heated are placed on trolleys, the tops of which are completely lined with firebricks. The underside of the trolley is protected from the high temperature in the upper parts by a sand seal.
It is also possible to use the furnaces for continuous operation. Material is charged from one end on trolleys and the trolleys are slowly pushed in. For annealing small castings or forgings smaller furnaces are operated with similar conveyors made of heat resisting alloys.
The electrical furnaces are best-suited for heat treatment purposes because of the possibility of accurate control of temperature in the furnace. In addition, it has the advantages of clean and easier operation, simplification of starting and closing down operation, less labour, no heat loss in flue gases, higher overall efficiency, possibility of heating in vacuum, ease of control of atmosphere. Electrical furnaces used for heat treatment purposes are generally of resistance type.
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In the resistance furnace, the external resistances are used which on getting heated up due to flow of electric current radiate heat to the charge in furnace. The furnace from inside is lined with firebricks and the resistors are made of either nichrome (upto 1300°C) or silicon carbide (up to 1450°C).
Points to be Considered during Designing Furnances:
In the design of these furnaces following points should be considered:
(i) The temperature range for the furnace-working must be such that the resistance wires will give long life when heated to 100°C above the furnace temperature.
(ii) The input rating should be known from the purpose to be achieved.
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(iii) The kilowatt loading per square metre of the heated furnace wall should be 25 to 30.
(iv) Usually wires from 2 to 5 mm diameter should be employed and used in the form of spiral. This accommodates long length of wires in short spaces. The diameter of the spiral should be 6 to 8 times the diameter of the wire and the spiral should be opened out so that there is at least 2-3 times thickness of wire between turns of the spiral.
(v) The time required for diffusion of heat into the charge varies with the material of the charge. For steels heat penetrates at the rate of 3 mm in 5 minutes. High carbon and alloy steels take a little longer to attain uniform temperature throughout.
(vi) Normally, the furnaces are built with automatic temperature controllers and time switches, so that very little attendance is required during normal operation.
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(vii) It may be noted that when silicon carbide rods are used as heating elements, the furnaces give much better life, if switching on and off is minimised. The heating elements age with intermittent use or wherever possible a low current should be maintained rather than switching off completely.
(viii) For heating of metallic wares it is not necessary to use muffles, but electric furnaces having protection for heating elements by winding them round a muffle may be used. In such cases it is important to note that the rate of heating must not exceed a critical value, otherwise the muffles will crack due to temperature stresses set up between the outside and the inside surfaces.
(ix) It is possible to have electric furnaces in which the heating element be at very high temperature and a blast of air be forced over the heating element which serves to carry away the heat quicker and also to cool the heating elements. The charge in such furnaces is heated both by conduction and radiation.
Types of Steel Heat Treating Furnaces:
Various types of steel heat treating furnaces heated by gas, oil or electricity, available commercially are described below in brief:
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I. Oven or Box Furnace:
This has a box or oven-shaped heating chamber. It is available in two versions (a) in-and- out type, and (b) continuous loading and unloading type. The in-and-out oven furnaces are loaded by hand or by a track- mounted car which, when rolled into the furnace, forms the bottom of the heating chamber.
The car type is used where heavy or bulky pieces must be handled. Some oven type furnaces are provided with a full muffle or semi-muffle which is an enclosed, refractory chamber into which the parts to be heated are placed. The full muffle being fully enclosed, prevents any flames or burning gases from coming in contact with the work and permits a special atmosphere to be used to protect or condition the work.
The semi-muffle, which is open at the top, protects the work from direct impingement of the flame although it does not shut the work off from the hot gases. In the direct-heat type oven furnace, the work is open to the flame.
In the electric oven furnace, a retort is provided if gas atmospheres are to be employed to confine the gas and prevent it from attacking the heating elements. Where muffles are used, they must be replaced periodically and a greater amount of fuel is required than in a direct heat type of the oven furnace.
For continuous loading and unloading, there are several types of furnaces, such as rotary car roller conveyor, furnace belt conveyor, walking beam or pusher-conveyor and a continuous-kiln type through which track-mounted cars are run. In the continuous type of furnace, the work must pass through several zones maintained at different temperature for preheating, heating, soaking and cooling.
II. Retort Furnace:
This is a vertical type of furnace provided with a cylindrical metal retort into which the parts to be treated are suspended either individually, if large enough, or in a container of some sort. The use of retort permits special gas atmospheres to be employed for carburising, nitriding, etc.
III. Pit Type Furnace:
This is a vertical furnace arranged for the loading of parts in a metal basket. The parts are heated by convection, the basket, when lowered into place, fits into the furnace chamber in such a way as to provide a dead-air space to prevent direct heating.
IV. Pot Type Furnace:
This furnace is used for the immersion method of heat-treating small parts. A cast-alloy pot is employed to hold a bath of molten lead or salt in which the parts are placed for heating.
V. Salt Bath Electrode Furnace:
In it, heating is accomplished by means of electrodes suspended directly in the salt bath. The patented grouping and design of electrodes provide an electromagnetic action which results in an automatic stirring action throughout the bath. This tends to produce an even temperature throughout.